1. Field of the Invention
The present invention relates to an optical waveguide.
2. Related Background Art
In recent years, optical waveguides have received a great deal of attention in various application fields. This is because use of an optical waveguide can provide a smaller and lighter optical system and eliminate the necessity of adjustment of the optical axis.
Optical waveguides are classified into a single mode waveguide, a double mode waveguide, and a multimode waveguide in accordance with a difference between the refractive index of a core and that of a clad, the width and depth of the waveguide, and a refractive index distribution. In a single mode waveguide, only a 0th-order mode is excited. In a double mode waveguide, two modes, i.e., 0th- and 1st-order modes are excited. In a multimode waveguide, three or more modes (0th-order mode, 1st-order mode, 2nd-order mode and more) are excited.
In an optical waveguide having a waveguide structure both in a direction of width and in a direction of depth, these modes are individually excited in the direction of width and in the direction of depth in the order of 0th-, 1st-, 2nd-order modes, etc.
Conventionally, in a known waveguide, a region having a high refractive index is formed by diffusing an element in part of a substrate, the region having a high refractive index is used as a core, and the substrate is used as a clad.
However, in the optical waveguide having a core formed by diffusing an element in part of the substrate, the width and depth of the core are determined by the diffusion rate of the element. For this reason, it is difficult to manufacture a waveguide having arbitrary maximum mode number in both the direction of width and the direction of depth.
For example, when diffusion progresses at the same rate in the direction of width and in the direction of depth, a waveguide having a narrow and deep core cannot be manufactured. It is thus hard to manufacture a waveguide having two modes in the direction of depth and one mode in the direction of width.
In applications of optical waveguides to various apparatuses, use of a double mode waveguide capable of utilizing the interference of the 0th- and 1st-order modes allows wider applications. However, because the maximum mode number in the width and depth directions of the waveguide can hardly be controlled, the waveguides cannot be easily applied to apparatuses.